Although it'd be a cool way to add additional things into the game, which I'd personally love to see, I doubt actually foresee it happening. As Xander pointed out, it has to be hard-coded into the games initially to support the downloads. Although it certainly has a lot of potentially, I feel like Game Freak/Nintendo have always focused on having a more full-fledged story and an endless amount of possibilities for continued play, which meant that DLC wasn't necessary. It was always more about connecting with others to enhance your game experience, not just add random content to extend the game. Plus, making DLC would take away from the effort of making the (hopefully) next games that would follow after X/Y.
Although I'd love to see DLC added to maybe add previous regions/a more expansive post-game experience, I'd also just be happy having that in the original cartridge. Gen V took a step in the right directions by having the Seven Sage hunt. I'd much rather having the standard post-game options, plus something (hopefully) more expansive than that to kind of wrap-up all the loose ends of the story.